<html lang="zh-Hans">
  <head><meta charset="utf-8"><title>calculator</title></head>
  <body>
    <form method="post">
      <input name="num1" value='1'>
      <select name="sign">
        <option value="+">+</option>
        <option value="-">-</option>
        <option value="*">*</option>
        <option value="/">/</option>
      </select>
      <input name="num2" value='1'>
      <button type="submit">Submit</button>
    </form>
    {{result}}
  </body>
</html>